This invention relates to a compound semiconductor device, and more particularly to a novel compound semiconductor device increased in the electron mobility, by alternately laminating In.sub.x Ga.sub.1-x As layer (0.7.ltoreq.x.ltoreq.1.0) and In.sub.y Ga.sub.1-y As layer (0.ltoreq.y.ltoreq.0.3) which differ significantly in the lattice constant, and doping n-type impurities only into the In.sub.y Ga.sub.1-y As layer (0.ltoreq.y.ltoreq.0.3), so as to realize a structure different from the existing lattice-matched alternate laminating structure of GaAs and GaAlAs or GaAs and AlAs (in both pairs, the lattice mismatching is about less than 0.3%; the lattice mismatching (%) is defined as {[(lattice constant of GaAlAs or AlAs)-(lattice constant of GaAs)].times.100}/(lattice constant of GaAs)].
As fabricating method of compound semiconductor devices, particularly electronic devices, the epitaxial growth method is generally employed owing to the thin and uniform growth of layer and ease of control of constituent element composition ratio. Above all, the molecular beam epitaxial (MBE) growth method is known as one of the particularly noticed techniques recently. For example, a device utilizing the MBE growth method and thin layer periodic structure is described in details by W. T. Tsang in Nikkei Electronics No. 308, 163 (1983).
According to this MBE growth method, the crystal growth speed can be controlled on a single atomic plane level (see J. P. van der Ziel et al., J. Appl. Phys., 48 (1977), p. 3018), and, furthermore, when combined with the reflective electron diffraction method, the composition of one atomic plane can be accurately controlled (see J. H. Neave et al., Appl. Phys. A 31, 1, 1983).
By employing such MBE growth method, it is possible to fabricate a high electron mobility transistor (HEMT) as shown in FIG. 3.
Incidentally, microwave elements using conventional compound semiconductors are disclosed, for example, in the Japanese Unexamined Patent Publication Nos. 4085/1984 and 147169/1983.
In the HEMT structure shown in FIG. 3, a GaAs layer 12 functioning as a buffer layer is formed on a semi-insulating GaAs substrate 11, and an undoped GaAs layer 13 serving as a channel layer is formed thereon. On the undoped GaAs layer 13 is formed an electron supply layer 14 having a high impurity concentration such as n-Ga.sub.x Al.sub.1-x As, and in the middle of the electron supply layer 14 there is a layer 15 made of a semiconductor containing p-type impurities at high concentration and possessing a large electrophilic ability, while a gate electrode 16 is disposed on this layer 15. Furthermore, the surface region 17 of the electron supply layer 14 at both sides of the layer 15 is alloyed, and a source electrode 18 and a drain electrode 19 are formed thereon.
In thus composed semiconductor device, when a proper bias voltage is applied to the gate electrode 16, a two-dimensional electron gas 20 is formed at the channel layer 13 side at the interface of the electron supply layer 14 and the channel layer 13. As a result, many electrons flow in the portion of a few tens angstrom units in thickness near the interface in the channel layer 13 where only few impurity ions are present. Therefore, scatter of impurity ions which is one of major causes of limiting the electron mobility is less, so that a high electron mobility may be realized.
The constitution in FIG. 3 makes use of the electron storage layer at the single hetero interface of GaAs layer and GaAlAs layer, whereas examples of making use of plural hetero interface of GaAs layer and GaAlAs layer by forming a multi-layer structure by alternately laminating GaAs layer and GaAlAs layer are described in details by R. Dingle et al. in Appl. Phys. Lett., 33, 665 (1978), and by T. J. Drumond et al., in J. Appl. Phys., 53 (2), 1023 (1982), and others.
In such compound semiconductor device, however, since the primary object lies in satisfaction of lattice matching condition on the hetero interface, that is, to control the lattice mismatching under 0.3%, there is a certain limit in increasing the degree of energy discontinuity at the hetero interface. For example, the relation between energy gap and lattice constant about generally known compound semiconductors is shown in FIG. 4. At the present, since only GaAs substrate and InP substrate are present as the semi-insulating compound semiconductor substrates, in order to match the lattice constant on the interface of the substrate and the epitaxial layer and on the hetero interface of the channel layer and the electron supply layer, conventionally, GaAs channel layer and Ga.sub.x Al.sub.1-x As or AlAs electron supply layer are formed on GaAs substrate, or In.sub.0.53 Ga.sub.0.47 As channel layer and In.sub.0.52 Al.sub.0.48 As supply layer are formed on InP substrate. In such structures, however, the difference of energy gap on the hetero interface between the channel layer and the electron supply layer is only about 0.7 eV at most (when using GaAs as the channel layer and AlAs as the electron supply layer). Still more, when the AlAs layer is doped with Si, a deep impurity level occurs, and in the structure as shown in FIG. 3, the composition rate y in Ga.sub.y Al.sub.1-y As is usually selected somewhere between 0.7 and 0.8. Actually, accordingly, the energy gap difference on the hetero interface between the channel layer (GaAs layer) and the electron supply layer (Ga.sub.y Al.sub.1-y As layer) is only about 0.2 eV.
Hence, in the conventional structure, if the lattice matching condition is satisfied on the interface between the semi-insulating substrate and the epitaxial layer and the hetero interface between the channel layer and the electron supply layer, it is evident that the degree of energy discontinuity is significantly limited. According to FIG. 4, aside from the combinations of GaAs-GaAlAs (or AlAs) and In.sub.0.53 Ga.sub.0.47 As-In.sub.0.52 Al.sub.0.84 As, it may be possible to use InP substrate, employing In.sub.0.53 Ga.sub.0.47 As layer as the channel layer and employing, for example, AlAs.sub.x Sb.sub.1-x having a large energy gap difference from In.sub.0.53 Ga.sub.0.47 As as the electron supply layer. But, generally, it is technically difficult to develop MBE growth of mixed crystal containing two elements of group V, and, still worse, since the compound AlAs.sub.x Sb.sub.1-x contains two elements, Al and Sb, which are easily oxidized, it is difficult to obtain a layer of high purity (the purity before Si doping), and handling of fine processing after growth is difficult. Not only AlAs.sub.x Sb.sub.1-x layer, but also three-element mixed crystal or four-element mixed crystal such as In.sub.0.53 Ga.sub.0.47 As or In.sub.0.52 Al.sub.0.48 As is generally complicated in composition control for lattice matching, and the growth technology is extremely difficult, which may bring about new problems, such as poor reproducibility and difficulty in unifying the composition distribution within wafer plane. Therefore, as the combination of two-element compound satisfying the lattice matching condition on the interface between the semi-insulating substrate and the epitaxial layer and on the hetero interface between the channel layer and the epitaxial layer, GaAs-AlAs (or Ga.sub.x Al.sub.1-x As) is studied predominantly. As for Ga.sub.x Al.sub.1-x As, as clear from FIG. 4, since the lattice mismatching between GaAs and AlAs is very small, about 0.3%, the lattice matching condition with GaAs is satisfied regardless of the composition rate x of Ga.sub.x Al.sub.1-x As, so that strict composition control is not required as compared with such combinations as In.sub.x Ga.sub.1-x As-In.sub.x Al.sub.1-x As etc. Hence, growth technology is not so difficult. However, in the combination of GaAs-Ga.sub.x Al.sub.1-x As (x=0.7 to 0.8), as mentioned above, the energy gap difference is limited up to about 0.2 eV. Thus, smallness of energy gap difference, that is, smallness of degree of energy discontinuity on the hetero interface induces to decrease the effect of electron confinement in the two-dimensional electron gas layer mentioned above, to lower the degree of formation of two-dimensional electron gas at room temperature, and to lower the electron mobility on the hetero interface.
In such conventional structure, the fact that the electron mobility in the two-dimensional gas layer decreases suddenly as approaching from low temperature to room temperature is described in details by T. J. Drummond et al., J. Appl. Phys., 53 (1982), 1023, and others.